V-ATPases are ubiquitous and highly conserved proton pumps responsible for organelle acidification in virtually all eukaryotic cells and for proton export in a few cell types. Through their roles in organelle acidification, V-ATPases impact macromolecular degradation, protein sorting, pH homeostasis, and sequestration of ions and nutrients. Recent data has revealed that V-ATPases play a central role in multiple pathophysiological conditions. For example, they help defend against some types of neurodegeneration, but can promote tumor metastasis and osteoporosis. They are thus attractive drug targets, but their complexity also makes them difficult. V-ATPases are multisubunit enzymes comprised of a peripheral complex, the V1 sector, attached to an integral membrane complex, the Vo sector; interaction between these two sectors is a major target of enzyme regulation. We propose to exploit the unparalleled flexibility of the yeast V-ATPase model system to address several issues that are critically important but experimentally intractable in mammalian V-ATPases. In Aim 1, we will test the hypothesis that the endosome/lysosome signaling lipid PI(3,5)P2 interacts directly with the Vo sector of the V-ATPase and regulates enzyme activity by stabilizing V1- Vo interactions. Depletion of this lipid iin mammals results in neurodegeneration; our experiments may indicate whether loss of organelle acidification is a potential cause. In Aim 2, we will use compartment-specific ratiometric fluorescent probes to test the contributions of two different yeast subunit isoforms to pH regulation in vivo. Higher eukaryotic cells often have several V-ATPase subunit isoforms whose individual contributions to organelle pH control and regulation are unclear; results from the more experimentally tractable yeast system may serve as a paradigm for isoform-dependent pH control in other cells. Finally, we have found that both acute and chronic loss of V-ATPase activity triggers downregulation of the major plasma membrane proton exporter, suggesting an unexpected level of coordination between the major organelle and plasma membrane pH control mechanisms. In Aim 3, we will test the hypothesis that loss of organelle acidification, possibly sensed at the endosome, induces ubiquitin-dependent internalization of proton export machinery at the plasma membrane as a compensatory mechanism. Mechanisms for balancing demands of organelle acidification, cytosolic pH control, and proton export are likely present in all cells but are largely unexplored. These experiments will begin to address how organelle acidification is sensed and preserved.